Gas turbine engines, such as those found on jet aircraft, comprise a housing within which fuel and air are combined and burned. The resulting hot exhaust gasses are used to turn a turbine and provide thrust when they exit the housing. Such engines generally include a fan to draw air into the housing and a compressor that compresses the air and sends it to a gas generator. Here a precisely metered supply of fuel is mixed with the compressed air and burned. The expanding exhaust gasses turn a turbine which powers at least the compressor and the fan. The exhaust gasses then pass through the remainder of the housing and exit the housing to provide thrust.
Such engines sometimes include sections called “afterburners” or “thrust augmentors” or merely “augmentors” that allow a gas turbine engine to produce additional thrust for limited periods of time—to help an aircraft take off, or during critical military maneuvers, for example. Augmentors may comprise one or more sets of fuel nozzles located in a chamber downstream from the gas generator in which chamber additional fuel is burned to increase engine thrust.
Fuel is often drawn from aircraft fuel tanks by pumps driven by the aircraft engines. Therefore, the amount of fuel pumped is proportional to the speed of the engines and not necessarily equal the amount of fuel required. Because of this difference, it is generally necessary to provide a fuel bypass recirculation system that comprises a pathway around which fuel is recirculated until it is needed. Pumping fuel through a bypass recirculation pathway, however, heats the fuel, and overheated fuel can cause problems. For example, overheated fuel is less able to perform a cooling function and may cause coking at the fuel nozzles. One way of cooling the fuel in the recirculation pathway is to return some portion of the pumped fuel to the aircraft fuel tank where it mixes with relatively cooler fuel. Cooler fuel is drawn from the fuel tank, and this process maintains the temperature of the fuel system at a reasonable temperature.
FIG. 2 illustrates a portion of a conventional aircraft fuel system including a fuel tank 200, a fuel pump 202 drawing fuel from fuel tank 200, and a fuel line 204 providing fuel to a first zone of augmentor nozzles 206, referred to herein as augmentor lightoff nozzles 206 and augmentor second zone nozzles 208. Fuel to the lightoff nozzles 206 is metered by an augmentor lightoff metering valve 210 and passes through a shutoff valve 212 between lightoff metering valve 210 and the lightoff nozzles 206. Fuel to the augmentor second zone nozzles 208 is metered by augmentor second zone metering valve 214, and passes through a shutoff valve 216 between the second zone metering valve 214 and the second zone nozzles 208.
The fuel system further includes a bypass system 220 through which a portion of the fuel is recirculated inversely proportional to the burn flow demands of the gas generator nozzles and the augmentor nozzles 206 and 208. Fuel in this pathway tends to heat up, and therefore a thermal recirculation flow metering valve 222 is provided for selectively returning a portion of the recirculating fuel to fuel tank 200 directly or, optionally, via a shutoff valve 224. This fuel mixes with relatively cool fuel in the fuel tank and results in a larger quantity of fuel being drawn from the fuel tank 200 by fuel pump 202 into the fuel system reducing the fuel system temperature. The operation of augmentor lightoff metering valve 210, augmentor second zone metering valve 214 and the thermal recirculation flow metering valve 222 is controlled by engine electronic controller 226.
The thermal recirculation flow metering valve and shutoff valve increase the weight and cost of a fuel system. It is generally desirable to lower the weight and/or cost of aircraft fuel systems.